IN Red and Rebol(3), you can use the split function to split a string into a list of items:
>> items: split {1, 2, 3, 4} {,} == ["1" " 2" " 3" " 4"] What is the corresponding inverse function to join a list of items into a string? It should work similar to the following:
>> join items {, } == "1, 2, 3, 4" There's no inbuild function yet, you have to implement it yourself:
>> join: function [series delimiter][length: either char? delimiter [1][length? delimiter] out: collect/into [foreach value series [keep rejoin [value delimiter]]] copy {} remove/part skip tail out negate length length out] == func [series delimiter /local length out value][length: either char? delimiter [1] [length? delimiter] out: collect/into [foreach value series [keep rejoin [value delimiter]]] copy "" remove/part skip tail out negate length length out] >> join [1 2 3] #"," == "1,2,3" >> join [1 2 3] {, } == "1, 2, 3" per request, here is the function split into more lines:
join: function [ series delimiter ][ length: either char? delimiter [1][length? delimiter] out: collect/into [ foreach value series [keep rejoin [value delimiter]] ] copy {} remove/part skip tail out negate length length out ] 
There is an old modification of rejoin doing that
rejoin: func [ "Reduces and joins a block of values - allows /with refinement." block [block!] "Values to reduce and join" /with join-thing "Value to place in between each element" ][ block: reduce block if with [ while [not tail? block: next block][ insert block join-thing block: next block ] block: head block ] append either series? first block [ copy first block ] [ form first block ] next block ] call it like this rejoin/with [..] delimiter
But I am pretty sure, there are other, even older solutions.
Following function works:
myjoin: function[blk[block!] delim [string!]][ outstr: "" repeat i ((length? blk) - 1)[ append outstr blk/1 append outstr delim blk: next blk ] append outstr blk ] probe myjoin ["A" "B" "C" "D" "E"] ", " Output:
"A, B, C, D, E" 
spaced: specialize 'delimit [delimiter: space]andunspaced: specialize 'delimit [delimiter: blank]. To make them more useful, the delimiting omits voids and blanks ("unset!" and "none!" in R3-Alpha and Red), then is smart about coalescing delimiters...so delimit ["a" if false ["b"] if false ["c"] "d"] {, } would be "a, d".